The ability to detect photons, and have accurate information on the magnitude of the flux, its arrival time and duration, is of critical importance to many applications. These range from biology, medicine, communications, environmental monitoring, particle and space physics to general instrumentation.
Each application enforces a different set of requirements on the photodetector element, ranging from high detection efficiency and large area detection to operation in magnetic environments. These requirements result in the many custom detector choices available on the market today ranging from solid state PIN and APD (Avalanche Photodiodes) devices to vacuum based photomultiplier tube (PMT) solutions.
In recent times, the dominance of PMTs is being challenged by the advance of silicon technologies and the development of novel detector devices. One in particular, termed here the Silicon Photomultiplier (SPM), has emerged as a direct competitor to the traditional PMT technology.
SPM devices were first developed in Russia in the mid-80's. Referred to in the literature as SiPM, MRS APD or SSPM, the first actual photodetectors based upon this structure were produced in 1989. The SPM is an extension of the concept of the Geiger-Mode avalanche photodiode (GAPD) which is reported widely in the literature, see for example (a) Z. Y. Sadygov et al., “Avalanche Semiconductor Radiation Detectors”, Trans. Nucl. Sci. Vol. 43, No. 3 (1996) 1009; and (b) V. Saveliev, “The Recent Development and Study of Silicon Photomultiplier”, Nucl. Instr. Meth. A 535 (2004) 528-532.
An SPM uses an array of photodiodes operating in Geiger mode with integrated quenching elements, summing the electrical output of all the diodes. The net result is a series of pulses (from the diodes that have detected a photon) being added together. As individual diodes detect photons the summed output will increase or decrease. This produces an analogue electrical output which is proportional to the number of photons incident on the total sensor. The gain in this case is still >105.
Compared with PMTs the technology is highly attractive in offering a low bias voltage, high gain, small form factor product which is not sensitive to magnetics or ambient light.
Despite early developments, the technology has been slow to evolve commercially because of the lack of early manufacturing developments. This has changed dramatically over the last 5 years with the arrival of several manufacturers producing SPM devices in volume.
SPM detector sizes on the market today are currently limited to detection areas of the order to several millimetres square. To compete with PMT, large area detection comparable to the typical PMT area of ˜1″ (2.5 cm) sizes and greater is required.
The techniques disclosed in WO 2006/126027 allow the tiling of SPM sensor elements into a larger array; this has allowed the production of silicon photomultiplier detectors with an active area of between 1 mm and 3 mm in diameter, for example. For larger areas SPM arrays, a flexible substrate is disclosed in WO 2008/052965, which allows the connection of the top contacts of the SPM arrays and allows the assembly of large numbers of SPM diodes to allow either a summed or pixellated output to be achieved.
In this context, a summed output is provided when multiple detectors are connected together at the output to produce a single larger area detector; when operating in such a mode, the sensor is not capable of imaging and can only detect light incident on it, not distinguish spatial information. On the other hand, a pixellated output is provided when multiple pixels in an array can be addressed and read out individually; when operating in such a mode, the sensor is capable of imaging.
SPM diodes are currently produced in diameters of between 1 mm and 3 mm. However, there is an increasing demand for detectors which are in the order of 1.2 cm in diameter. This can either be accomplished by manufacturing a single large piece of silicon with 1.2 cm diameter, or by tiling together an array of e.g. 3 mm diameter detectors in a 4×4 array (see, for example, WO 2006/126027 and WO 2008/052965). The output for these arrays varies from summed to pixellated and it is necessary to provide a common framework under which both those two concepts can work.